The chairman of the US House of Representatives Intelligence Committee has flatly denied Donald Trump’s claims about a wiretap on his Trump Tower residence in New York.
But, speaking at a hearing on Russia's involvement in last year’s presidential election, he said it was still possible other surveillance was used against Mr Trump.
In his opening statement, Republican Devin Nunes told the committee: "Let me be clear. We know there was not a wiretap on Trump Tower.
“However, it's still possible that other surveillance activities were used against President Trump and his associates."
The Republican also said the committee has seen no evidence to date that officials from any campaign conspired with Russian agents, but will continue to carry out investigations into the issue.
He also said the committee will investigate who has been leaking classified information about investigations into Russia's interference.
Mr Nunes said he hopes the committee's hearings will result in a “definitive report” on Russia's involvement in the presidential election.
FBI director James Comey and National Security Agency Director Michael Rogers will both testify.
Mr Trump, who recently accused former president Barack Obama of wiretapping his New York skyscraper during the campaign, took to Twitter before the hearing began, accusing Democrats of making up allegations about his campaign associates' contact with Russia during the election.
He said Congress and the FBI should be going after media leaks, and maybe even Hillary Clinton, instead.
Mr Trump tweeted: “The real story that Congress, the FBI and others should be looking into is the leaking of Classified information. Must find leaker now!”
Mr Trump also suggested, without evidence, that Mrs Clinton's campaign was in contact with Russia and had possibly thwarted a federal investigation.
US intelligence officials have not publicly raised the possibility of contacts between the Clintons and Moscow.
Officials investigating the matter have said they believe Moscow had hacked into Democrats' computers in a bid to help Mr Trump's election bid.
Monday's hearing, one of several by congressional panels probing allegations of Russian meddling, could allow for the greatest public accounting to date of investigations that have shadowed the Trump administration in its first two months.
US intelligence agencies concluded that Russia meddled in the campaign to help Mr Trump defeat his Democratic rival Mrs Clinton. The FBI has also been investigating ties between Russia and Trump advisers and associates during the campaign.
Associated Press contributed to this report